In recent years, as a fixing device included in an electrophotographic image forming apparatus such as a copying machine and a printer, a fixing device using a belt fixing method has increasingly been used. In this method, a fixing belt is held around a fixing roller and a heat roller, and the fixing roller and a pressing roller are pressed against each other via the fixing belt (see Patent Literature 1).
In the fixing device using the belt fixing method, the fixing belt having a heat capacity smaller than a heat capacity of the fixing roller is heated. Thus, the fixing device has an advantage of a short warm-up time as compared to a fixing device in which a fixing roller and a pressing roller directly abut each other. In addition, the fixing device using the belt fixing method does not require a heat source such as a halogen lamp inside the fixing roller. This allows the fixing roller to be provided with a thick elastic layer which is made of, e.g., sponge rubber and which has a low hardness. Consequently, it is possible to advantageously secure a large nip width.
However, in the case where the fixing device using the belt fixing method has a fixing roller provided with an elastic layer having a large thickness and a low hardness, since the fixing roller cannot contain a heat source, it is impossible to heat the fixing roller in a state where the fixing belt is not being rotated. Consequently, in a case where continuous printing is started in a state where the fixing roller is not heated sufficiently, e.g., immediately after a warm-up or when the image forming apparatus is ready on standby, heat of the fixing belt is rapidly drawn to the fixing roller, so that the fixing belt is caused to have a temperature lower than a fixing temperature. In other words, an undershoot occurs. This problematically leads to defective fixing.
To solve this problem, according to a conventional fixing device using the belt fixing method, the fixing belt is driven to rotate even during a warm-up or when the image forming apparatus is ready on standby. This allows heat of the heat roller to be provided to the fixing roller via the fixing belt, so that the fixing roller is heated.
However, in the case where the fixing belt of the conventional fixing device using the belt fixing method is driven to rotate, respective abutting surfaces of the fixing roller and the fixing belt slip on each other due to rigidity of the fixing belt. This makes it difficult to appropriately rotate the fixing belt only by driving the fixing roller to rotate. Thus, it has been necessary to (i) press the fixing roller and the pressing roller against each other via the fixing belt so that the fixing belt is sandwiched between the two rollers, and in this state to (ii) drive both of the pressing roller and the fixing roller to rotate so that the fixing belt is rotated. This method, however, has caused problems (1) and (2) below.
(1) Since the pressing roller is driven to rotate while being constantly pressed with great force against a surface of the fixing belt during a warm-up or when the image forming apparatus is ready on standby, the fixing belt is easily deteriorated, and thus has a shortened life. In particular, an image forming apparatus for normal use is ready on standby for a period far longer than a period during which the image forming apparatus is carrying paper. Therefore, the above rotation under pressure affects the fixing belt significantly.
(2) The pressing roller is pressed against the fixing belt that has a set temperature higher than that of the pressing roller. This excessively raises a temperature of the pressing roller. Thus, even if the pressing roller is provided with a heat source and a temperature sensor so as to control the temperature of the pressing roller, it is still difficult to control the temperature so that the pressing roller has a predetermine temperature.
Patent Literature 2 discloses, as a technique for solving the above problems, a fixing device including an auxiliary roller for driving a fixing belt to rotate.